In the last decade fiber optic sensors have gained considerable acceptance as alternatives to conventional sensing technologies. Fiber optic sensors provide several advantages over conventional sensors, including higher sensitivity, freedom from electromagnetic interference, and multiplexing capabilities. More recently, Fiber Bragg Gratings (FBGs) have emerged as an extremely versatile sensing technology. The Bragg resonance wavelength is sensitive to external perturbations through the therm-optic and stress-optic effects. As a result, FBGs can be used to sense a variety of environmental parameters such as temperature, strain, and pressure. In most applications either strain or temperature, but not both, is the parameter of interest.
One drawback of FBGs as sensor elements is that temperature and strain are often indistinguishable, because the change in the Bragg resonance wavelength is the result of a linear combination of strain and temperature effects. To overcome this ambiguity, a technique has been developed by Xu et al. which relies on the dispersion of the stress- and therm-optic coefficients. The technique utilizes two FBGs with different grating periods. Measurement of the temperature and strain dependence of the two Bragg wavelengths produces a system of two equations and two unknowns. These two equations can then be solved using matrix methods to determine the temperature and strain present in the surrounding medium. See M. G. Xu, J. L. Archambault, L. Reekie, and J. P. Dakin, "Discrimination Between Strain and Temperature Effects using Dual-Wavelength Fiber Grating Sensors," Elec. Lett. 30 (13) 1085 (1994).
One drawback of the technique used by Xu et al. is that the inverse of the 2.times.2 matrix must be well-conditioned. Thus, a large wavelength separation between the Bragg wavelengths of the FBGs comprising the sensor must be used. Additionally, the matrix elements must be known with a high degree of accuracy to minimize the error in temperature and strain recovery. Recently, Patrick et al. proposed a hybrid FBG/ Long Period Grating (LPG) sensor for strain/temperature discrimination. The sensor uses two FBGs in combination with a single LPG to simultaneously measure both temperature and strain. A detailed description of this sensor can be found by reading H. J. Patrick, G. M. Williams, A. D. Kersey, J. R. Pedrazzani, and A. M. Vengsarkar, "Hybrid Fiber Bragg Grating/ Long Period Fiber Grating Sensor for Strain/Temperature Discrimination," IEEE Phot. Tech. Lett. 8 (9)1223 (1996).
The sensors described by Xu et al. and Patrick et al. are examples of state-of-the-art fiber grating based temperature and strain sensors. However, these sensors have numerous deficiencies. First, both sensor designs require a complicated fabrication procedure. The dual-wavelength concept described by Xu et al. requires two FBGs to be fabricated per sensor while the hybrid sensor described by Patrick et al. requires three gratings per sensor (2 FBGs and 1 LPG ). In particular, the hybrid sensor requires all three gratings to be positioned accurately in wavelength. Second, both sensors make the design of multiplexed (or multi-point) sensor systems complicated. The dual-wavelength sensor requires the determination of two separated Bragg wavelengths (for example, 1.3 and 1.5 .mu.m ) per sensor, and the hybrid sensor requires the determination of a single Bragg wavelength and two reflected amplitudes per sensor. Additionally, the latter occupies approximately 30-40 nm of spectral bandwidth making the multiplexing of a large number of sensors difficult at best. Finally, since the sensor described by Patrick et al. uses FBGs to probe the loss spectrum of the LPG, the shift of both the LPG and FBGs are encoded in the magnitudes of the reflected signals. As a result, processing of the received signals is difficult and still dependent on matrix techniques to finally derive the strain and temperature information. Furthermore, a review of the sensor literature demonstrates that the sensor element and multiplexing architecture are typically considered on a independent basis.
In view of the disadvantages in the art, it would be desirable to provide a sensor system for sensing at least one physical parameter employing a long-period grating, which provides simplified processing of the signals received from the long-period grating and facilitates the multiplexing of a large number of sensors. It would also be desirable to provide a sensor system which has the ability to athermalize the LPGs in the system and a system concept which considers the multiplexing system and sensor elements in a unified manner.